With the remarkable development of small, thin, and high-functionality portable electronic equipment in recent years, there is a growing demand for smaller, thinner, and high-capacity batteries as their power source. Lithium batteries are effectively used for this purpose because they can be designed small with high capacity, and in particular, flat prismatic lithium ion rechargeable batteries are suitable in making the equipment thinner, for which they have been increasingly used as the repeatedly usable battery for portable electronic equipment such as mobile phones.
Meanwhile, various structures for sealing a prismatic sealed battery have been known, in which a sealing member is placed via a gasket at the opening of a battery case, and the opening of the battery case is crimped inwards to seal the case while compressing the gasket (for example, see Japanese Patent Laid-Open Publications Nos. Hei 8-162076 and 2000-357495).
One example of the structure of this type of a sealed battery is described with reference to FIG. 6: The battery case 21 accommodates elements for electromotive force 22 composed of an electrode plate assembly and liquid electrolyte. An annular groove is formed in the outer face at a predetermined position from the top end of the opening 21a of the battery case 21, and a holder plate 24 formed with an aperture 24a is welded to be fixed onto an annular support 23 that is formed by the annular groove protruding inwards. A plate 26 is placed upon the holder plate 24 via an insulating gasket 25, and one electrode of the elements for electromotive force 22 is connected to the plate 26 through a lead 27, while a cap 28 that serves as an external connection terminal is placed on and electrically connected to the sealing plate 26. The top end of the opening 21a of the battery case 21 is crimped inwards in this state so as to secure the plate 26 via the insulating gasket 25 and to seal the case. The insulating gasket 25 is square tubular and includes a bottom wall 25a that makes contact with the lower face of the plate 26 and a top wall 25b that makes contact with the inner periphery of the opening 21a of the battery case 21, the bottom wall 25a having an aperture 25c that matches the aperture 24a of the holder plate 24 and an annular sealing protrusion 29 along the outer periphery in the inner surface thereof. The plate 26 is formed with a thin part 26a that breaks when the internal pressure of the battery rises, and the cap 28 that serves as the external connection terminal is formed with a gas outlet hole 28a. 
A battery with a sealing unit 30 instead of the plate 26 of FIG. 6, as shown in FIG. 7, is also known. The sealing unit 30 includes a filter 31 and an inner gasket 32 placed inside the filter, the inner gasket being square tubular and including a bottom wall 32a, a top wall 32b, and an aperture 32c formed in the bottom wall 32a. A safety mechanism 35 is further provided, which consists of a pair of an upper valve 33 and a lower valve 34 respectively disposed above and below the bottom wall 32a of the inner gasket 32, the pair of valves having cup-shaped parts 33a and 34a that are connected to each other at their middle parts within the aperture 32c. The cup-shaped part 34a of the lower valve breaks when the internal pressure of the battery rises, which causes the cup-shaped part 33a of the upper valve to be inverted to cut off the current path. Upon the safety mechanism 35 are placed a PTC element 36 and a cap 37 that serves as an external connection terminal. The opening 31a of the filter 31 is crimped inwards in this state so as to secure the cap 37, the PTC element 36, and the safety mechanism 35 via the inner gasket 32 to complete the sealing unit 30.
The structures shown in FIG. 6 and FIG. 7, however, have the following problems: In a thin battery with a small thickness W of the battery case 21, the thickness of the top wall 25b of the insulating gasket 25, or of the top walls 25b and 32b of the insulating gasket 25 and the inner gasket 32, which will be the compressed point, cannot be made sufficiently large. This means sufficient space necessary for the compressing is not available when crimping the opening 21a of the battery case 21, or the openings 21a and 31a of the battery case 21 and the filter 31, inwards to compress the insulating gasket 25, or the insulating gasket 25 and the inner gasket 32, between the opening and the top face of the cap 28 or 37 that serves as the external connection terminal. Sufficient space necessary for the compressing is not available even when the dimension d1 between the annular support 23 and the top end face of the battery case 21, or the dimension d1 and the dimension d2 between the upper end face and the lower end face of the filter 31 are controlled when crimping the battery case 21, or the battery case 21 and the filter 31. It is therefore often the case that sealing properties are not satisfactory, resulting in low reliability in respect of leakage resistance.
In view of the above-described problems in the conventional technique, an object of the present invention is to provide a sealed battery which has high sealing properties with a simple and low-cost structure even in a thin design and which is highly reliable in respect of leakage resistance.